1. Technical Field
The present invention relates to the field of optical probe surface inspection by interferometry, and in particular to a method and apparatus for fine-controlling the position of a predetermined probe location relative to a fixed reference point of a probe processing apparatus fixedly coupled to an auxiliary optical laser apparatus in which method the position is controlled with optical means.
2. Description of the Related Art
One prior art system is disclosed in U.S. Pat. No. 5,469,259 to IBM Corporation, Armonk, USA. In the disclosed system a surface profile interferometer is used as a device for determining the roughness of a surface or the height of a step change in the thickness of a part being measured. Such a step change may be caused, for example, by the application of a metal film to a substrate in the manufacture of a printed circuit board or an integrated microcircuit. In general terms, an interferometer is an optical instrument in which two beams of light derived from the same monochromatic source are directed along optical paths of different length, in which the difference in length determines the nature of an interference pattern produced when the light beams are allowed to interfere. Since the beams of light are derived from the same monochromatic source, they are identical in wavelength. At equal path distances from the source, they are also in phase with one another. Phase differences between the beams therefore result only from differences in path length.
The phenomenon of light wave interference results from the mutual effect of two or more waves passing through the same region at the same time, producing reinforcement at some points and neutralization at other points, according to the principle of superposition.
With a photoelectric shearing interferometer, the height of a step change in a test surface may be measured using polarized light passed through a slit, through a Wollaston prism, and through a microscope objective lens, to form two images of the slit, with one image on each side of the step change. The beams reflected by the test surface pass through the lens and the prism, with an image being formed by two orthogonally polarized beams. The phase difference between these beams, which is determined by the height of the step, may be measured by the linear movement of a weak lens in a lateral direction (transverse to the beam) until the phase difference is exactly cancelled, as determined by the use of an electro-optic modulator, an analyzer, a photomultiplier, and a phase-sensitive detector, which are used together to detect the phase equality of the two interfering beams. The accuracy of the system depends on the precision to which the linear movement of the weak lens can be measured.
In the above-referenced United States patent, a separate autofocus system is required for maintaining the focus of the main imaging path of the interferometer. This is done by a separate arrangement according to confocal technique, i.e. to control the intensity of a target spot and maintaining the intensity at the maximum level. A disadvantage is that too many optical elements are used which makes the autofocus system difficult to adjust and renders it error-prone.
It is thus an objective of the present invention to provide a method and respective system for positioning a predetermined probe location in an automated way and avoiding the disadvantages of confocal autofocus systems.